念のため、以前も同様の観察が報告されており、一時的な状況として説明されたものと同様の現象です（5 ヶ月前）：
Rake error during update (not fatal)
Docker アップグレード中：
Waiting for Unicorn to reload.......................
$ bundle exec rake assets:flush_sw
rake aborted!
Don't know how to build task 'assets:flush_sw' (See the list of available tasks with `rake --tasks`)
Did you mean? assets:clean
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/exe/rake:27:in `\u003ctop (required)\u003e'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `\u003cmain\u003e'
(See full trace by running task with --trace)
WARNING: Unable to flush service worker file
Running post deploy migrations
Discourse アップグレード中：
Waiting for Unicorn to reload..........................
$ bundle exec rake assets:flush_sw
rake aborted!
Don't know how to build task 'assets:flush_sw' (See the list of available tasks with `rake --tasks`)
Did you mean? assets:clean
/var/www/discourse/vendor/bundle/ruby/2.6.0/gems/rake-13.0.1/exe/rake:27:in `\u003ctop (required)\u003e'
/usr/local/bin/bundle:23:in `load'
/usr/local/bin/bundle:23:in `\u003cmain\u003e'
(See full trace by running task with --trace)
WARNING: Unable to flush service worker file
Running post deploy migrations